The invention relates to a fuel injection system for mixture compressing externally ignited internal combustion engines which employ continuous fuel injection into the induction tube. In engines of this type, the induction tube contains an air flow rate meter as well as a throttle valve which can be actuated arbitrarily by the operator. A restoring force opposes the force of the air on the air flow rate meter and the air flow rate meter displaces a valve slide within a fuel metering system and thereby meters out fuel in proportion to the air flowing through the induction tube. The restoring force acting on the air flow rate meter is a pressurized fluid, the pressure of which may be changed especially in dependence on temperature by means of a control pressure valve.
It is the purpose of fuel injection systems of this type automatically to provide a favorable fuel-air mixture for all operational states of the engine so as to insure complete fuel combustion and the highest possible power and/or lowest fuel consumption. At the same time, it is desired to reduce as much as possible the generation of toxic exhaust gas constituents. To achieve these various and conflicting purposes, the fuel quantity must be very precisely metered out in accordance with the requirements of each and every engine state.
In known fuel injection systems of this type, the fuel quantity is metered out as nearly as possible in proportion to the air flow rate through the induction tube. The ratio between the metered out fuel and air may be changed by changing the restoring force acting on the air flow rate meter in dependence on operational variables of the engine and by means of at least one control pressure valve.
It is known that when the engine temperatures are lower than approximately 30.degree. C., a reliable engine start is obtained only if additional fuel is provided to the engine. This additional fuel is delivered in known fuel injection systems by means of a starting device which is turned on substantially at the time the engine is started and which consists mainly of an electromagnetic fuel injection valve and a thermal switch that limits the opening time of this valve and/or cuts it off entirely at higher temperatures. The electromagnetic starting valve injects the additional fuel into the induction manifold. The thermal switch either opens or closes the circuit of the valve in dependence on engine temperature. When an engine start takes place at a temperature below approximately 30.degree. C., the cold starting circuitry is interrupted by means of an electrically heated bimetallic contact.
A known cold starting device of this type requires additional expenditure because of the supplementary electromagnetic starting valve and the thermal switch and, in addition, the injection of raw fuel into the induction manifold results in a relatively poor distribution of that fuel to the various cylinders. Furthermore, due to the relatively long time required for the effects of the additional injection to be felt and terminated, the fuel enrichment is substantially greater than actually necessary, resulting in rough idling, a high toxic emission factor and high fuel consumption.